Clarify the difference between Jupyter Book and the MyST engine and when to use each

[choldgraf]

Our JB docs could make it easier to answer a few questions:

• What's the relationship between jupyter book and mystmd at a technical level?
• What user archetype is each meant for? When should you use one vs. the other?
• What's the short-hand way for describing this relationship? (e.g. "Jupyter Book is a distribution of the MyST Engine" vs. "Jupyter Book is MyST")

Some easy-to-find guidance (perhaps on the ecosystem page, or the landing page) would help.

[zmoon]

It would also be useful to have some guidance to help decide whether an issue should be posted here or at mystmd (or somewhere else like the theme repos). (Does that already exist somewhere?)